Method and apparatus for growing crystals



Sept. 11, 1962 R. T. DOLLOFF METHOD AND APPARATUS FOR GROWING CRYSTALS Filed Feb. 11, 1959 RICHARd F B EZ OFF M W 42 BY %A 72 i Z ATTORNEY 3,053,639 Patented Sept. 11, 1962 This invention relates to improved methods and apparatus for growing crystals at high temperatures using the Verneuil fusion method.

The Verneuil fusion method consists in starting crystal growth on a crystal the upper part of which is kept at a high temperature sufficient to produce a thin molten layer thereon. Powdered feed material for growing the desired crystal is placed in a vertical container above the growing crystal, and is allowed to fall thereon.

In the above described method, heat is provided to the crystal by means of radio frequency heaters, by a furnace, a flame, or an arc. Such heating means require the use of special containers for the crystal. In addition, where an oxyhydrogen flame or a direct are are employed, considerable impurities may be introduced into the material being melted. Less objectionable heat sources such as radio frequency require that the sample be conductive.

With a view to overcoming the above outlined limitations of the Verneuil method and apparatus, the main object of this invention is to provide crystal growing means characterized by intense clean heat together with controlled atmosphere and the elimination of special containers for the crystal sample.

A related object of this invention is to provide apparatus of the character described, which employs a high output carbon are heat source suitably disposed with respect to a growing crystal, and with respect to a system of plane or concave mirrors.

These and related objects, features and advantages of the present invention will be more fully understood as the description thereof proceeds, particularly when taken in conjunction with the accompanying drawing wherein FIG. 1 is a cross-sectional elevational View of crystalgrowing apparatus in accord with the invention; and FIG. 2 is a schematic view depicting an arc image furnace containing the apparatus shown on FIG. 1.

Broadly construed, the apparatus of the invention comprises a transparent enclosure having crystal support means, gas inlet and gas outlet means, and powder feed means. A source of radiant energy is associated by means of mirrors with the above apparatus in such a way that energy can be reflected and refocused to an image on the crystal support at some distance from the source. Viewed in its broadest aspect, the method of the invention comprises placing a seed or pseudo seed crystal on suitable crystal support means, placing the support at the image spot of a mirror, supplying the mirror with energy from a radiant energy source, feeding crystal growth material on the surface of the seed, melting the material, cooling the formed crystal, and removing the cooled crystal from its support.

Referring to the drawing, FIG. 1 shows details of a crystal growth apparatus, in accord with the invention. The apparatus described comprises a transparent enclosure Suitably made of quartz or other like material, and having outlet means 12 at its bottom. As will be observed on FIG. 2, enclosure It] depends from the top 14 of a chamber 16, which can be evacuated or filled with inert gases. Enclosure 10 has a flared-out top section, which cooperates with base members 18 and 20 to secure the enclosure in place, and in air-tight engagement with the lower extremity of crystal feed mechanism 30.

Depending'from member 24 and extending into enclosure 10 is an adjustable crystal support 24, in this example a bent high melting point support having a flattened-out extremity or pedestal 26, which may be water-cooled, if

desired. The pedestal is so positioned with respect to concave mirror 27 surrounding enclosure 10 as to lie at its image spot.

Depending from the top of chamber 22 is a crystal feed mechanism comprising a funnel-shaped member 28 provided with a gas-tight cover 40and gas inlet 38, and having an elongated tapered extremity 30 passing through member 20 and extending immediately above crystal support 26. A suitable rapping mechanism 32 communicates with perforated shaker 34 through cover 40 in such manner that feed material contained in shaker 34 is delivered at a controllable rate through the tapered tube 30 to the growing crystal supported by pedestal 26.

The crystal growing assembly of the invention is shown in its complete form on FIG. 2. The assembly comprises a radiation chamber 36 mounted at a suitable distance from crystal-growing chamber 16.

As shown, chamber 36 contains a concave mirror 48 at the near focus of which is secured at water-cooled, high output illuminating carbon are 35. The are image is transferred, in the manner shown by arrows in the drawing, through openings 37 and 39 in chambers 36 and 16, respectively, to a water-cooled plane or corrected plane mirror 41 mounted at an angle of 45 in chamber 16. Concave mirror 27 is vertically positioned with respect to mirror 41, so that in accord with well known laws of optics the radiant heat is concentrated on the growing crystal. Energies of 1500 watts per centimeter squared over a 1 centimeter squared area can be obtained are symmetrically heated as they fall toward the image. Y

Upon reaching the image area, the particles are melted by the intense radiant energy, and finally grow as a crystal or crystals on the seed surface. If desired, a rod of the material to be melted may be substituted for the powdered form, and may be fed into the image spot at a suitable regulated rate.

Since the seed is of the same composition as the material being melted, there is no need for selection of a suitable crucible to withstand the very high temperatures employed here. The top surface of the seed is maintained in the molten state; therefore, the only material with which the growing crystal comes in contact is that of its own composition. Such a construction permits the growth of crystal either under vacuum or under pressure, as desired.

The instant apparatus and method are particularly suitable for the preparation of single crystals of materials such as molybdenum. Since molten molybdenum is quite reactive particularly at the temperatures reached in crystal growing, it is most difficult to prepare pure single crystals of this material. Other workers have grown molybdenum crystals by introducing the metal in some form directly into the arc stream, and thus melting it. Again this technique may result in some contamination of the crystal and necessarily involves the use of a gaseous atmosphere in the crystal-growing region. The use of a vacuum, as is possible with the instant apparatus, is, therefore, impossible with prior art techniques.

As illustrative of the practice of the present invention, high quality single crystals of molybdenum were grown. One of the crystals measured /8 inch in length by 1 inch in diameter, and was of 99.9-lpercent purity, and

was free from inclusions and grain boundaries. The crystal growing operation was carried out for periods of about five minutes at are currents approaching 290 amperes in an atmosphere of argon. Feed rates were about mm? per minute. Initial melting began at about 220 amperes, and the current was steadily increased up to 290 amperes during the remainder of the experiment.

This method and apparatus of the invention may also be used to prepare high purity crystals of compounds formed by direct reaction of a refractory powder with a selected atmosphere in the region of the image. By the use of high purity refractory metal powders such as Ti and Zr, and a purified atmosphere of nitrogen, the production of single crystals of TiN and ZrN of purities one or two orders of magnitude better than those now common in the field becomes an easy matter. Thus, single crystals of AlN have been prepared by the reaction of Al powder in a nitrogen atmosphere with subsequent crystal growth. These crystals measured 1 mm. in length by 0.01 mm. in diameter. Their purity is high, as is indicated by their transparent nature. Doping agents may be added to a material such as AlN to provide the energy absorption necessary for the growth of larger crystals. This would, of course, cause some reduction in purity of the finished crystal, but such material would still be useful for many purposes.

.Stoichiometry of the crystals of compounds formed by the method and apparatus of the invention may be closely controlled by adjustment of the composition and pressure of the atmosphere in the growth chamber, and also by adjustment of the composition of the feed material. For example, in growing TiB a small excess of boron may be added to the feed material to counteract the preferential loss of boron from the melt, if stoichio-metric or boron-rich TiB crystals are desired. Alternatively, an excess of titanium in the feed material would result in titanium-rich TiB crystals.

Many other materials may be melted and crystals grown therefrom by this technique. A few examples are silicon, titanium, vanadium, germanium, tungsten carbide, silicon carbide, titanium carbide, vanadium carbide, molybdenum carbide, zirconium diboride, titanium diboride, tungsten boride, zirconium nitride and tantalum nitride. This method may be used to prepare crystals from materials having melting points much lower than 2000 C. to 3600 C., such as were discussed earlier. Metals or compounds melting at 1000 C. or slightly lower can be employed if suitable controls such as filters 4 or shutters are incorporated to regulate the intensity, and spectral distribution and exposure time of radiation on the sample.

What is claimed is:

1. Apparatus for growing crystals from materials having melting points up to 3600 C., comprising a transparent enclosure having crystal support means, gas inlet and outlet means, crystal growing feed means positioned above said crystal support means, and a source of radiant energy associated with said enclosure by means of mirrors such that energy can be reflected and refocused on said crystal support means.

2. Apparatus for reacting materials having melting points up to about 3600 C. and for growing crystals therefrom, comprising a transparent enclosure surrounded by having a first concave mirror, crystal support means mounted at the image spot of said mirror, gas inlet and outlet means, crystal growing feed means positioned above said crystal support means, a plane mirror mounted vertically below said support means facing said concave mirror and in reflecting relationship therewith, and a source of radiant energy mounted at the near focus of a second concave mirror in reflecting and refocusing relationship with said plane mirror.

3. The apparatus of claim 2 wherein said plane mirror is mounted at a 45 angle.

4. The apparatus of claim 2 wherein said crystal support means are water-cooled.

5. The apparatus of claim 2 wherein said plane mirror is water-cooled.

6. The apparatus of claim 2 wherein said source of radiant energy consists of a water-cooled high output illuminating carbon arc.

References Cited in the file of this patent UNITED STATES PATENTS 1,379,187 Kaufman May 24, 1921 2,461,019 Alexander Feb. 8, 1949 2,634,554 Barnes Apr. 14, 1953 2,872,292 Altmann Feb. 3, 1959 2,907,642 Rumrnel Oct. 6, 1959 FOREIGN PATENTS 774,270 Great Britain May 8, 1957 OTHER REFERENCES Pfann: Zone Melting, April 1958, pages 78 and 79. 

1. APPARATUS FOR GROWING CRYSTALS FROM MATERIAL HAVING MELTING POINTS UP TO 3600* C., COMPRISING A TRANSPARENT INCLOSURE HAVING CRYSTAL SUPPORT MEANS, GAS INLET AND OUTLET MEANS, CRYSTAL GROWING FEED MEANS POSITIONED ABOVE SAID CRYSTAL SUPPORT MEANS, AND A SOURCE OF RADIANT ENERGY ASSOCIATED WITH SAID ENCLOSURE BY MEANS OF MIRRORS SUCH THAT ENERGY CAN BE REFLECTED AND REFOCUSED ON SAID CRYSTALS SUPPORT MEANS. 